There are many scientific and industrial applications which require ultraviolet (UV) laser light with good beam quality. Such applications include, but are not limited to, irradiating masks for integrated circuit fabrication, micro-machining of high-density semiconductor memory devices, drilling precisely controlled holes in multi-layer circuit boards and manufacturing fiber Bragg gratings (FBGs). However, no laser apparatus presently used for such applications is entirely satisfactory.
FBGs are portions of optical waveguides, such as optical fibers, which have been processed to reflect and transmit specific wavelengths. Producing FBGs involves exposing the fiber to UV light, the intensity of which varies between light and dark along the length of the fiber. The light and dark bands of exposure are placed along the fiber with spacing comparable to the wavelength of light to be reflected by the fiber in operation. The UV light induces changes in the index of refraction of the fiber, producing an index grating along the length of the fiber.
A light source used for exposure of a fiber to make FBGs must provide light within specific ranges of wavelengths in the UV portion of the spectrum. A typical fiber""s primary wavelength range for absorption peaks near 240 nanometers, and wavelengths differing from the peak by more than about 10 nanometers are significantly less effective. Even at the peak wavelength, only a small fraction of the laser power is absorbed, so it is highly desirable for the light source to provide light at a wavelength near the absorption peak.
The current sources of UV light used for the previously-described applications have various drawbacks. For example, most current production systems for FBGs and integrated circuits use excimer lasers. Excimer lasers can produce output beams with high average powers, which facilitates processing, but they have serious disadvantages. They require toxic, corrosive gases for operation, have high operating and maintenance costs, and produce short duration (xcx9c50 nanoseconds), low repetition rate ( less than 1000 hertz), high peak output power (xcx9c1 Megawatt) UV pulses. The output beams of excimer lasers have relatively poor beam quality. Moreover, the high peak output powers of excimer lasers cause damage to optical fibers, weakening them and making them susceptible to fracture.
Several other types of lasers have been used for integrated circuit manufacturing and FBG production, but none are entirely satisfactory. For example, ion lasers provide good beam quality at a number of wavelengths in the UV, but have high operating costs, produce weak output and are very inefficient. The output beams of frequency-doubled ion lasers typically have powers of one-half watt or less, poor beam quality, and short operational lifetimes. The output wavelength of frequency-doubled copper vapor lasers is slightly too long to be optimal. Liquid dye lasers are impractical for large-scale industrial production since they require frequent changes of the liquid dye solution to maintain operation.
Solid-state lasers which include active laser media such as neodymium-doped yttrium aluminum garnet (Nd:YAG), Nd:LiYF4 (Nd:YLF), Yb:YAG, Yb: phosphate glass, etc., can be efficient and can provide output power with good beam quality. Incorporating a diode pump can result in a compact laser device. The most efficient output of such lasers lies in the infrared portion of the spectrum near 1064 nanometers and 1319 nanometers. Accordingly, fundamental wavelengths of greater than 1 micron are selected for high-power applications.
The infrared output of diode-pumped lasers can be efficiently converted to the green portion of the spectrum by nonlinear frequency conversion such as second harmonic generation (SHG) or frequency doubling. A number of crystalline materials are used for nonlinear frequency conversion, such as lithium niobate, lithium triborate (LBO), or potassium titanyl phosphate (KTP).
However, subsequent conversion of these visible outputs into the UV by sum-frequency-mixing or frequency-doubling is typically less efficient. This stems primarily from the properties of the nonlinear optical materials used for UV light generation: LBO, xcex2-barium borate (BBO), and cesium lithium borate (CLBO).
For these and other reasons, prior art UV laser systems suffer from relatively low conversion efficiency and/or poor beam quality. Moreover, none of these prior art lasers can efficiently provide a high-quality output beam having a wavelength near 240 nanometers, which would be suitable for FBG production and other applications.
The present invention provides devices and methods for efficiently producing output beams with wavelengths near 240 nanometers from solid state lasers.
In one preferred embodiment, the diode-pumped infrared solid-state laser is a Nd:YAG laser operating on the 946 nanometer line, rather than the more commonly-used 1064 nanometer line. The 946 nanometer wavelength is frequency-doubled to 473 nanometers in the blue using periodically-poled potassium titanyl phosphate (PPKTP), LBO, or a similar nonlinear crystal. The 473 nanometer blue light is converted to an ultraviolet fourth harmonic beam using a non-critically phase matched (NCPM) CLBO crystal.
The use of the 946 nanometer infrared laser wavelength provides for enhanced UV light generation using non-critical phase-matching in CLBO, and this combination provides for efficient overall generation of 236 nanometer UV light.
In some embodiments, the first CLBO crystal is cooled to between xe2x88x9210 degrees centigrade and xe2x88x9220 degrees centigrade. In some embodiments, the first CLBO crystal is disposed in a container of dry inert gas such as nitrogen, dry air, helium, neon, argon, krypton or xenon. In other embodiments the first CLBO crystal is disposed in a vacuum.
In some embodiments, UV light with wavelengths below 237 nanometers is generated by sum-frequency mixing using NCPM CLBO. In some such embodiments, a rare earth doped garnet laser is tuned to emit a second fundamental beam at a wavelength of approximately 1077 nanometers, the second fundamental beam and the fourth harmonic beam are directed to a second cesium lithium borate crystal for sum-frequency mixing to produce an output beam at approximately 194 nanometers.
In other embodiments of the present invention, an apparatus for generating ultraviolet light includes: a garnet crystal doped with a rare earth element; a diode pump laser for pumping the garnet crystal; a resonator for generating a fundamental beam having a wavelength of approximately 946 nanometers from the pumped garnet crystal; a periodically-poled potassium titanyl phosphate crystal for producing a second harmonic beam having a wavelength of approximately 473 nanometers; and a CLBO crystal cooled to a temperature in the range from xe2x88x9210xc2x0 centigrade to xe2x88x9220xc2x0 centigrade and oriented for non-critical phase-matching, for producing a fourth harmonic beam having a wavelength of approximately 237 nanometers.
In some such embodiments, the garnet crystal is a neodymium-doped yttrium aluminum garnet crystal. The neodymium-doped yttrium aluminum garnet crystal may include a first un-doped end portion, a doped central portion and a second un-doped end portion.
In yet other embodiments of the present invention, an apparatus for generating ultraviolet light includes: an Nd:LiYF4 laser tuned to output a fifth harmonic at approximately 209 nanometers; a garnet laser doped with a rare earth element and tuned to output a fundamental beam at approximately 1305 nanometers; and a CLBO crystal for sum-frequency mixing the fundamental beam and the fifth harmonic beam to produce an output beam at approximately 180 nanometers.